Chapter 16 : Interference and Diffraction 16-1: Interference
Light Waves Combine with Each Other
Interference only takes place between waves with the same wavelength.
If two waves with the same wavelength cross each other's pathe, they combine to form a resultant wave.
This process is called interference.
Monochromatic - Light waves with the same wavelength.
There are two types of interference: Constructive and Destructive
Destructive Interference - The resultant amplitude is less than the amplitude of the larger component wave.
When this occurs there is a dark spot that exists where the interference is occuring.
Constructive Interference - The component waves combine to make a new wave with greater amplitude than
either of the original two waves. When this occurs the resultant light is brighter than the two previous waves.
Waves must have a constant phase difference for interference to occur. If the crest of one wave overlaps the crest of another wave,
the two have a phase difference of zero degrees and are said to be in phase. If the two waves have a phase difference of 180 degrees
they are said to be out of phase.
Coherency
When the phase difference between two waves is constant and the waves do not shift relative to each other as time passes, the waves
are said to have coherence.
When the waves from two light sources interfere and the conditions change with each phase change this is known as incoherency.
Interference can be demonstrated through pasing a single light source through a narrow slit and then through two narrow slits. This process
produces a series of brigh and dark parallel bands (fringes).
This behavior can be used to calculate various tendencies of the behavioral patterns of light. The behavior can be used to determine wavelength
by measuring the distance of the fringes and the angle they make with the slit. There are two equations, one for constructive and one for destructive interference.
EQUATION FOR CONSTRUCTIVE INTERFERENCE
EQUATION FOR DESTRUCTIVE INTERFERENCE
16-2: Diffraction
Diffraction - Light deviating from a straight-line path and enter the region that would otherwise be shadowed, a divergence of light from its initial
path or direction of travel.
This behavior, diffraction, most often occurs wehn waves pass through small opening, around obstacles, or by sharp edges.
Diffraction may resemble interference based on the patterns of light that result.
Huygen's principle states that each portion of a slit acts as a source of waves. Therefore light from one portion of the slit can interfere with
light from another portion.
Fringe patterns resulting will often look like a wave dispersing after a rock is dropped in a pond.
Light diffracted by an obstacle also produces a diffraction pattern.
Diffraction will sometimes separate white light into its component colors. Scientists use spectrometers to evaluate this behavior.
Below is a diagram of diffraction. 16-3: Lasers
Lasers and Coherence
Laser - A device that produces an intense, nearly parallel beam of coherent light.
Noncoherent light, such as that of an incandesant light light bulb, will disperse in all directions and light a general area. However, light emitted from
a laser concentrates the lightwaves and makes coherent and focused light. The way this process occurs is the mechanisms within a laser convert
energy into coherent light. They use a substance called an active medium.
They high powered and coherent light emitted from LASERS has various applications in the real world. It helps with data transfer and storage as well as
communication. Lasers have also been used to correct poor vision in more modern applications of lasers in medicine.
REVIEW QUESTIONS
16-1:
1.) What is the necessary condition for a path length difference between two waves that interfere
constructively? destructively?
A 1: The peaks of each wave must be in alignment so that the waves be constructive. To be destructively interfering it must be in opposite manner.
2.) If the distance between two slits is .0550mm, find the angle between the first-order and second-order bright fringes for yellow light with a wavelength of 605nm.
A 2: Use the equation provided above. The answer is 27°
16-2:
1.) A point source of light is inside a container that is opaque except for a single hole. Discuss what happens to the image of the point source projected onto a screen as the hole's width is reduced.
A: The process of diffration occurs because this behavior scatters the light and waves of the light source.
2.) Light passes through a diffraction grating with 3550 lines/cm and forms a first-order maximum at an angle of 12.07°.
a. What is the wavelength of the light?
b. At what angle will the second maximum appear?
A: a. 21.2 m
b. 81.9°
16-3:
1.) How does light from a laser differ from light whoe waves all have the same wavelength but are not coherent?
A: LASERS use a medium and technology to covert energy into highly concentrated and coherent light. This process does not occur in other light sources such as an incandescant light bulb.
2.) Fiber-optics systems trasmit light by means of internal reflection within thin strands of extremely pure glass. In these fiber-optic systems, laser light is used instead of white light to transmit signal. Explain why.
A: Because the coherent light of a LASER focuses the light specifically and allows for greater precision to hone in the signal for the various processes
Holt, Rinehart and Holt Physics. New York: Holt, Rinehart & Winston, 2001.
16-1: Interference
Light Waves Combine with Each Other
- Destructive Interference - The resultant amplitude is less than the amplitude of the larger component wave.
When this occurs there is a dark spot that exists where the interference is occuring.- Constructive Interference - The component waves combine to make a new wave with greater amplitude than
either of the original two waves. When this occurs the resultant light is brighter than the two previous waves.- Waves must have a constant phase difference for interference to occur. If the crest of one wave overlaps the crest of another wave,
the two have a phase difference of zero degrees and are said to be in phase. If the two waves have a phase difference of 180 degreesthey are said to be out of phase.
Coherency
- When the phase difference between two waves is constant and the waves do not shift relative to each other as time passes, the waves
are said to have coherence.Interference can be demonstrated through pasing a single light source through a narrow slit and then through two narrow slits. This process
produces a series of brigh and dark parallel bands (fringes).
This behavior can be used to calculate various tendencies of the behavioral patterns of light. The behavior can be used to determine wavelength
by measuring the distance of the fringes and the angle they make with the slit. There are two equations, one for constructive and one for destructive interference.
EQUATION FOR CONSTRUCTIVE INTERFERENCE
EQUATION FOR DESTRUCTIVE INTERFERENCE
16-2: Diffraction
Diffraction - Light deviating from a straight-line path and enter the region that would otherwise be shadowed, a divergence of light from its initial
path or direction of travel.
- This behavior, diffraction, most often occurs wehn waves pass through small opening, around obstacles, or by sharp edges.
- Diffraction may resemble interference based on the patterns of light that result.
- Huygen's principle states that each portion of a slit acts as a source of waves. Therefore light from one portion of the slit can interfere with
light from another portion.Light diffracted by an obstacle also produces a diffraction pattern.
Diffraction will sometimes separate white light into its component colors. Scientists use spectrometers to evaluate this behavior.
Below is a diagram of diffraction.
16-3: Lasers
Lasers and Coherence
Laser - A device that produces an intense, nearly parallel beam of coherent light.
Noncoherent light, such as that of an incandesant light light bulb, will disperse in all directions and light a general area. However, light emitted from
a laser concentrates the lightwaves and makes coherent and focused light. The way this process occurs is the mechanisms within a laser convert
energy into coherent light. They use a substance called an active medium.
They high powered and coherent light emitted from LASERS has various applications in the real world. It helps with data transfer and storage as well as
communication. Lasers have also been used to correct poor vision in more modern applications of lasers in medicine.
REVIEW QUESTIONS
16-1:
1.) What is the necessary condition for a path length difference between two waves that interfere
constructively? destructively?
A 1: The peaks of each wave must be in alignment so that the waves be constructive. To be destructively interfering it must be in opposite manner.
2.) If the distance between two slits is .0550mm, find the angle between the first-order and second-order bright fringes for yellow light with a wavelength of 605nm.
A 2: Use the equation provided above. The answer is 27°
16-2:
1.) A point source of light is inside a container that is opaque except for a single hole. Discuss what happens to the image of the point source projected onto a screen as the hole's width is reduced.
A: The process of diffration occurs because this behavior scatters the light and waves of the light source.
2.) Light passes through a diffraction grating with 3550 lines/cm and forms a first-order maximum at an angle of 12.07°.
a. What is the wavelength of the light?
b. At what angle will the second maximum appear?
A: a. 21.2 m
b. 81.9°
16-3:
1.) How does light from a laser differ from light whoe waves all have the same wavelength but are not coherent?
A: LASERS use a medium and technology to covert energy into highly concentrated and coherent light. This process does not occur in other light sources such as an incandescant light bulb.
2.) Fiber-optics systems trasmit light by means of internal reflection within thin strands of extremely pure glass. In these fiber-optic systems, laser light is used instead of white light to transmit signal. Explain why.
A: Because the coherent light of a LASER focuses the light specifically and allows for greater precision to hone in the signal for the various processes
Holt, Rinehart and Holt Physics. New York: Holt, Rinehart & Winston, 2001.